Due to the focus of on environmental protection, efforts are ongoing with respect reduction of emissions from marine engines. This also involves the steady optimization of lubrication systems for such engines. Thereto is added increased competition and the economic aspects of reducing oil consumption, as this is a significant part of the operational costs of ships. A further concern is proper lubrication despite reduced lubricant because the longevity of diesel engines should not be compromised by the reduction of oil consumption. Thus, there is a need for steady improvements with respect to lubrication.
For lubricating of large slow-running two-stroke marine diesel engines, several different systems exist, including injection of lubrication oil directly onto the cylinder liner or injection of oil quills to the piston surface.
An alternative method, commercially called Swirl Injection Principle (SIP), is relatively new and based on injection of a spray of atomized droplets of lubrication oil into the scavenging air swirl inside the cylinder. The helically upwards directed swirl results in the lubricant being pulled towards the Top Dead Centre (TDC) of the cylinder and pressed outwards against the cylinder wall as a thin and even layer. This is explained in detail in International Patent Application WO2010/149162. The lubricant injectors are non-return valves that comprise an injector housing inside which a reciprocating valve member is provided, typically a valve needle. The valve member, for example with a needle tip, in a valve housing closes and opens the lubricant's access to a nozzle aperture according to a precise timing. In current SIP systems, a spray with atomized droplets is achieved at a pressure of 35-40 bars, which is substantially higher that the oil pressure of less than 10 bars that are used for systems working with compact oil jets that are introduced into the cylinder. In some types of SIP valves, the high pressure of the lubrication oil is also used to move a spring-loaded valve member against the spring force away from the nozzle aperture such that the highly pressurised oil is released therefrom as atomized droplets. The ejection of oil leads to a lowering of the pressure of the oil inside the valve member, resulting in the valve member returning to its origin until the next lubricant cycle where highly pressurized lubrication oil is supplied to the lubricant injector again.
In such large marine engines, a number of injectors are arranged in a circle around the cylinder in a plane perpendicular to a cylinder axis and each injector comprises one or more nozzle apertures at the tip for delivering lubricant jets or sprays into the cylinder from each injector. Examples of SIP lubricant injector systems in marine engines are disclosed in international patent applications WO02/35068, WO2004/038189, WO2005/124112, WO2010/149162, WO2012/126480, WO2012/126473, and WO2014/048438, which are herewith incorporated by reference.
Traditionally, the cylinders of engines have been built with openings for oil injectors placed at a distance from the top dead centre (TDC) of the cylinder, where the distance about ⅓ or more of the total stroke of the cylinder. However, for increasing length of the cylinders, considerations apply whether the lubrication nozzles should be moved further towards the TDC in order to safeguard a proper lubrication in the cylinder near the TDC, where the heat is high and where the requirements for proper lubrication are most critical. For oil quills that are applied to the piston directly, such considerations have been disclosed by Miyake et al. in the article “PAPER NO.: 177 Cylinder liner and piston ring lubrication issues in relation to increase stroke/bore ratio”, published by the International Council on Combustion Engines at the CIMAC Congress 2013 in Shanghai. In these experiments, it was found that the positioning of the lubrication valves at large distance of 1.2 m from the TDC resulted in the oil not being scraped into the combustion chamber. From the perspective of oil refreshment and neutralization of sulphuric acid in the combustion chamber, re-positioning of the lubrication valves closer to the TDC at 0.3 m was found advantageous, as 67% of the oil would be scraped into the combustion chamber. However, for overall lubrication of the entire cylinder, two-level lubrication improved the situation much with 20% of the oil scraped into the combustion chamber.
As compared to scraping of oil quills into the upper part of the cylinder, special considerations apply for SIP lubrication because part of the spray from the SIP valve nozzles is drawn helically upwards towards the TDC and into the combustion chamber and therefore provides a better lubrication even over a large distance from the SIP nozzles to the TDC. This is also why changing from quill scraping of oil to SIP lubrication has proven general improved conditions. For these reason also, it is generally not believed that a moving of the SIP injectors to a position closer to the TDC would yield any improvement that would justify the modification of cylinder liners. Especially, an improvement as it was found by Miyake et al. for the re-positioning of the oil quill lubrication valves cannot be expected. Thus, although, one may speculate in moving the quill nozzles or the jet nozzles of more traditional lubrication systems further towards the TDC, these considerations do not appear to apply for lubricant sprays of SIP lubrication principles due to the oil-transporting helical swirl.
However, despite these apparent advantages of SIP lubrication systems, a general steady motivation for improvements exists.